The present invention relates to infrared analyzers which can be used in a variety of industrial and commercial applications. In particular the present invention utilizes an acousto-optic tunable filter which is operable in the near and mid-infrared spectral regions, which permits construction of a compact-high-throughput infrared spectral analyzer controlled by a microcomputer.
A large and growing market exists for analytical devices which can be used to analyze reaction products of a wide variety of industrial processes. In addition on-line real time combustion product analyzers are needed to facilitate more efficient burning of hydrocarbon fuels. Multi-function combustion product analyzers are needed to keep track of combustion emission and to ensure that the emission is within the limits set by environmental legislation. On-line multifunction analyzers are needed in the chemical and petroleum industries to function as process analyzers to facilitate process control systems. The petroleum industry as well as the emerging synthetic fuel industry has need for compact stable, simple analytical instruments for determining hydrocarbon fractions.
The analytical devices to date utilize ultraviolet and infrared spectrophotometry, as well as gas and liquid chromatography in meeting the laboratory and industrial needs outlined above. Such optical instruments as well as the system of the present invention utilize the following important characteristics of materials. A particular molecule has a characteristic absorption spectrum which is dissimilar from that of all other molecules. The spectra of mixtures of molecules are additive and the absorption is proportional to the concentrations of the molecules. Optical absorption spectra can be obtained from any type of sample be it solid, liquid, or gas so long as the sample is optically transmissive, and the spectra can be obtained in a non-destructive testing of the sample.
Infrared radiation is particularly suited for analyzing complex streams of hydrocarbons and combustion products because of the infrared absorption characteristics of the major gases produced in such systems. The existing infrared analytical systems typically are limited to measuring a single wavelength of interest at a given time. Such infrared instruments utilize prisms, selectively absorptive filters, or diffraction gradings as the filtering mechanism can require mechanical changing of the optical filter element in order to function over a variety of wavelengths. Such mechanical changeovers require realignment of the systems which are time consuming and difficult to achieve at an on-site location.
It has recently been recognized that certain birefringent optical materials which are termed acoustooptic materials can be used as a filter in a spectrum analyzer. In such acousto-optic materials, a light beam propagating as an E-ray can under certain conditions be converted into an O-ray by interation with, and diffraction from, an acoustic wave propagating from the same medium. This phenomenon has been used in fabricating narrow band optical filters, the peak transmission wavelength of which can be selected by properly choosing the frequency of the acoustic wave. Even more recently, new efficient infrared transmissive acousto-optic materials such as thallium-arsenic-selenide, as described in U.S. Pat. No. 3,792,287 owned by the assignee of the present invention, provide the possibility of operation over the near to mid-infrared spectrum from about 1 micrometer to about 16 micrometers.